linux-kernel - Re: [Bug 11219] KVM modules break emergency reboot

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Date:	Mon, 18 Aug 2008 16:03:55 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...ranet.com>
Cc:	bugme-daemon@...zilla.kernel.org,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [Bug 11219] KVM modules break emergency reboot


* Avi Kivity <avi@...ranet.com> wrote:

> (copying mingo)
>
> (context: sysrq-B with kvm-intel.ko loaded doesn't work. on my machine,  
> it kills the sata interface, but the processor and network keeps working)
>
> Strangely, the specs say:
>
>> • The INIT signal is blocked whenever a logical processor is in VMX  
>> root operation.
>> It is not blocked in VMX non-root operation. Instead, INITs cause VM  
>> exits (see
>> Section 21.3, “Other Causes of VM Exits”).
>
> So INIT (which is wired to the triple-fault processor output, it seems,  
> rather than RESET) is blocked and the machine is not reset completely.
>
> So we need to disable vmx during native_machine_emergency_restart().  
> There are at least three ways of doing this:
>
> - add a vmxoff sequence (with an exception handler) to  
> native_machine_emergency_restart(). while simplest, this will not  
> unblock INIT for other cpus
>
> - add an emergency_restart notifier_block, and have kvm subscribe. This  
> has the disadvantage of being slightly complex, opening a tiny race  
> (emergency restart during kvm module initialization), and requiring IPIs  
> during emergency restart.
>
> - move vmxon/vmxoff management out of the kvm module and into x86 core.  
> Bloats the core but reduces complexity. IPIs still required.
>
> I think the notifier block is the way to go. Ingo, let me know what you  
> prefer.

notifier should be OK i think - sysrq-b is an emergency mechanism after 
all.

btw., "echo b > /proc/sysrq-trigger" never worked reliably for me with 
KVM also loaded.

	Ingo
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